Optical fibers have now replaced copper wire as the preferred medium for carrying telecommunications signals. In this context, as with copper wire, it is necessary to provide both for interconnects and for efficient terminations onto devices. With the increasing trend towards miniaturization and integration, optical components (which may include both active and passive components) are more and more being integrated on optical chips as a single module for fiber interconnection. As a consequence, the termination problem increasingly becomes one of efficiently and reliably coupling optical fibers to the waveguide channels of planar lightguide circuits (PLCs). This requires the fiber and waveguide to be arranged, both in proximity and alignment with each other, in a manner that provides for efficient (low-loss) coupling of transmission signals from the fiber to the waveguide or vice versa. More generally, multiple waveguide channels need to be respectively connected to multiple fibers such that the cross-sectional pattern of PLC waveguide channels and the arrangement of the optical fibers are aligned with each other in a manner that minimizes the coupling loss from each output.
Owing to the very small cross-sectional sizes of the light-carrying channels in both the fiber and the PLC waveguide (typical linear dimensions being in the range of 2 to 10 microns), the orientational and positional adjustments entailed in optimizing the transparency of the connection call for great precision. Conceptually, the simplest way of accomplishing it is to use a micromanipulator simply to maneuver the fiber inside a bead of epoxy resin placed between the fiber and waveguide until the optical power transmitted through the connection is maximized. When optimum coupling is achieved, the epoxy can then be UV or thermally polymerized to complete the attachment. Though simple and economical, this method suffers from several drawbacks. Because the epoxy must have low optical absorption and be index matched to the fiber and waveguide, the selection of epoxies to make the joint are very limited in number and cannot be independently optimized for strength or other mechanical and thermal properties. In addition, in applications where the fibers and waveguides carry high optical powers or optical wavelengths capable of degrading the epoxy through heating and photochemical reactions, the long-term reliability of the joint is compromised.
These shortcomings have been met by anchoring the fiber to an alignment member, which most frequently takes the form of a supporting V-shaped groove prepared either in the surface of the PLC itself or in the surface of a separately prepared ‘V-block’. The general approach is to arrange the fiber in the aligning V-groove (which has been carefully prepared by etching techniques) and adjust and eventually lock its position so that the core of the fiber finely tunes to the geometric center of the corresponding waveguide. The most general alignment of a V-block with respect to a PLC channel involves six V-block variables: three translational (x,y,z) and three rotational (pitch, roll and yaw). Clearly, such a full alignment procedure can be carried out only for a fully and independently adjustable V-block. If, as is commonly the case, the V-groove is formed on the PLC itself (as in European Pat. No. 1015921 by M. R. J. Richard et al., and U.S. Pat. No. 5,175,781 by B. D. Hockaday et al., U.S. Pat. No. 5,600,745 by D-S Wuu et al., U.S. Pat. No. 5,784,509 by T. Yamane et al., U.S. Pat. No. 6,212,320 by A. G. Rickman et al., and U.S. Pat. No. 6,324,323 by V. Benham and H. Hatami-Hanza,) or the separately prepared V-chips are connected to the PLC by rods or by a plate (as in U.S. Pat. No. 5,297,228 by H. Yanagawa et al., U.S. Pat. No. 5,361,382 by S. Nakamura et al., and U.S. Pat. No. 5,703,973 by S. C. Mettler and I. A. White), then the alignment procedure is necessarily a much restricted one, usually concerning only two of the six variables: namely translational motions along the V-groove and along the axis of V-symmetry normal to the groove.
In many cases, commonly referred to as cases of ‘static alignment’, the accuracy of alignment for the fiber and the corresponding waveguide is completely predetermined by the design of the structure (such as the depth and angle of the V-grooves, positioning of alignment rods and locking plates, etc.) and no additional refinements are made optically prior to securing the elements. In other cases, which may termed examples of ‘limited active alignment’, measurements are taken of the optical transmission between the fiber and waveguide after initial alignment, and adjustments are made in order to provide a dynamic refinement of the coarser predetermined positions (see for example U.S. Pat. No. 5,175,781 by B. D. Hockaday et al. and U.S. Pat. No. 6,324,323 by V. Benham and H. Hatami-Hanza). These examples, however, like all those involving V-grooves formed on, or anchored to the PLC chip, are readily susceptible to active refinement only in the two translational directions set out above.
It is now generally accepted that active alignment procedures allowing for complete (six parameter) angular and translational adjustment produce interconnects with the lowest optical loss. In this context, a common procedure begins by polishing the mating faces of the independent V-block and PLC to a common small angle (conventionally about 8 degrees) away from the normal to the longitudinal axis of the fiber/waveguide system. This is done in order to prevent back reflections from the interface, which are generally detrimental to all optical device operations. After active alignment has been achieved, a process that typically brings the separation distance between the mating surfaces closer than 10 microns, a small quantity of adhesive (typically UV or thermal epoxy) is wicked into the gap and cured in order to secure the interconnect. As a consequence of the large area of the total mating surface (compared to the fiber and waveguide core cross-sections) the movement induced by epoxy polymerization disturbs primarily only the distance normal to the mated faces, which is the degree of freedom that least perturbs the previously minimized loss of the interconnect. Any small perturbations of the other degrees of freedom can be even further diminished by increasing the areas of the mating faces by using a wider V-block and correspondingly widening the PLC by affixing adjoining pieces to its top and bottom surfaces.
The primary drawback of the aforementioned active alignment procedure is that the adhesive that wicks into and eventually secures the joint also enters the light path between the fiber and waveguide cores and does so in a manner that is uncontrollable, irreproducible and probably non-uniform. Since no adhesive is completely transparent to light, the non-uniformity, development of strain during polymerization, lack of reproducibility, and optical degeneration of the adhesive in the presence of high temperature or high-power pump energy, are all detrimental to the general optical performance of the interconnect. An important improvement would therefore derive by excluding adhesive from the light path while maintaining the other aspects and integrity of the joint.